[llvm] AMDGPU: Update live intervals in convertToThreeAddress (PR #104610)

[Matt Arsenault via llvm-commits]

================
@@ -3947,14 +3947,32 @@ MachineInstr *SIInstrInfo::convertToThreeAddress(MachineInstr &MI,
       (ST.getConstantBusLimit(Opc) > 1 || !Src0->isReg() ||
        !RI.isSGPRReg(MBB.getParent()->getRegInfo(), Src0->getReg()))) {
     MachineInstr *DefMI;
-    const auto killDef = [&]() -> void {
+    const auto killDef = [&](SlotIndex NewIdx) -> void {
       const MachineRegisterInfo &MRI = MBB.getParent()->getRegInfo();
       // The only user is the instruction which will be killed.
       Register DefReg = DefMI->getOperand(0).getReg();
+
+      if (LIS) {
+        LiveInterval &DefLI = LIS->getInterval(DefReg);
+        LiveRange::Segment *OldSeg = DefLI.getSegmentContaining(NewIdx);
+
+        if (OldSeg->end == NewIdx.getRegSlot()) {
+          DefLI.removeSegment(OldSeg->start, NewIdx.getRegSlot(), true);
----------------
arsenm wrote:

We should actually be trimming the segment, we still want to leave the dead def behind 

https://github.com/llvm/llvm-project/pull/104610